1. Field of the Invention
The present invention relates generally to optically active devices, and more particularly to a single polarization optical fiber laser or amplifier.
2. Technical Background
Rare-earth doped fiber lasers such as ytterbium-doped fiber lasers are finding uses in such areas as materials processing, product marking and engraving, and micromachining. Ytterbium-doped fiber lasers operating with high-power, narrow linewidth, and high pulse energy are being developed. The application space of fiber lasers would be broadened by the availability of additional operating wavelengths and even higher output powers. Each could be achieved through nonlinear wavelength conversion and the coherent combination of several fiber lasers, respectively. Linearly polarized output, required for many of these applications, has been studied to a lesser extent than other attributes of fiber lasers.
Hence, it may be useful and even necessary in certain applications to have emission available that is linearly polarized in a stable direction of polarization for lasers and amplifiers. For linear or single polarization, it is desirable to obtain an optical polarizing (PZ) fiber which receives randomly elliptically polarized input light and provides output light polarized only along a single polarization. The polarization characteristic (single polarization) propagates one, and only one, of two orthogonally polarized polarizations while suppressing the other polarization by increasing its transmission loss. Such single polarization fibers generally have an azimuthal asymmetry of the refractive index profile. Single polarization optical fibers are useful for ultra-high speed transmission systems or for use as a coupler fiber for use with and connection to optical components (lasers, EDFAs, optical instruments, interferometric sensors, gyroscopes, etc.). Single polarization or linearly polarized lasers can be used to obtain an emission of a linearly polarized transversal monomode light wave useful in a large variety of fields. These fields include telecommunications, optical transmission, instrumentation, spectroscopy, medicine, the detection of chemical species and telemetry. Similarly, a linearly polarized fiber amplifier (LPFA), instead of a conventional Erbium Doped Fiber Amplifier (EDFA), when all or a portion of the PZ fiber doped with a rare-earth dopant is optically pumped, has significantly higher gain for one linear polarization state than for the orthogonal state for use with fiber-optic gyroscopes, interferometric fiber sensors, nonlinear frequency conversion, polarization multiplexing, and most designs of phase or amplitude modulators, as some more specific examples. By having such polarized fiber lasers or amplifiers, increased scaling power is achievable by employing known polarization beam multiplexing (PBM) to combine two beams into a single output having different polarization modes as long as they are orthogonally polarized.
Slight improvement in the polarization performance of single mode optical waveguides has been achieved by elongating or distorting the fiber core symmetry as a means of decoupling the differently polarized waves. However, the noncircular geometry and the associated stress-induced birefringence alone are, generally, not sufficient to maintain the desired single polarization for use as an improved fiber laser or fiber amplifier or their polarization beam multiplexing for improved power scaling.
It has, therefore, been an area of ongoing development to obtain a fiber laser or fiber amplifier providing single polarization that is maintainable and sufficient for power scaling through PBM.
Furthermore, linear single-polarization (SP) fiber laser oscillators or amplifiers that are robust and stable to external perturbations are required. By robust and stable it is meant devices that maintain single-linear polarization.
There is also a need to make SP fiber laser oscillators or amplifiers with large effective area for high power applications in order to avoid non-linear effects such as Raman and Brilluoin scattering.
There is also need to have stable linear SP fiber oscillators and amplifiers for optical power scalability using coherent-beam combination techniques.
Finally, SP fiber laser oscillators and or amplifiers are needed in order to avoid—at high output powers—temporal instabilities caused by non-linear coupling of co-propagating orthogonal polarization modes.